Solid-state imaging device and camera

ABSTRACT

A solid-state imaging device including is provided. The solid-state imaging device includes: pixels arrayed; a photoelectric conversion element in each of the pixels; a read transistor for reading electric charges photoelectrically-converted in the photoelectric conversion elements to a floating diffusion portion; a shallow trench element isolation region bordering the floating diffusion portion; and an impurity diffusion isolation region for other element isolation regions than the shallow trench element isolation region.

CROSS REFERENCES TO RELATED APPLICATIONS

The present application is a Continuation application of applicationSer. No. 12/003,981, filed Jan. 4, 2008, and contains subject matterrelated to Japanese Patent Application JP 2007-036620 filed in theJapanese Patent Office on Feb. 16, 2007, the entire contents of whichare incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a solid-state imaging device and acamera, and particularly relates to a MOS (metal-oxide semiconductor)solid-state imaging device and camera.

2. Description of the Related Art

Solid-state imaging devices include a charge-transfer solid-stateimaging device represented by a CCD (charge-coupled device) image sensorand an amplification solid-state imaging device represented by a MOS(metal-oxide semiconductor) image sensor such as a CMOS (complementarymetal-oxide semiconductor) image sensor. When comparing the CCD imagesensor with the MOS image sensor, the CCD image sensor may need a highdriving voltage to transfer signal electric charges, so that a powersupply voltage for the CCD image sensor may be higher than that of theMOS image sensor.

Accordingly, a mobile phone unit incorporating a camera, a PDA (personaldigital assistant) and other mobile devices typically use a CMOS imagesensor as a solid-state imaging device mounted thereon. The CMOS imagesensor is advantageous in that a power supply voltage is lower than thatof the CCD image sensor and power consumption is lower than that of theCCD image sensor.

For insulating and isolating elements, a LOCOS (local oxidation ofsilicon) (selective oxidation) element isolation system or a STI(shallow trench isolation) element isolation system is known as anelement isolation system used in the MOS image sensor (see JapaneseUnexamined Patent Application Publication No. 2002-270808). Inparticular, the STI element isolation system has been widely used withpixels increasingly miniaturized.

In a solid-state imaging device, the number of pixels has been increasedalong with the resolution being improved, and a pixel is furtherminiaturized because the solid-state imaging device includes a largenumber of pixels.

SUMMARY OF THE INVENTION

Since the pixel is increasingly miniaturized as the number thereof isincreased in the MOS image sensor as described above, the area of aphotodiode serving as a photoelectric conversion portion is reduced,with the result that a saturated electric charge amount and thesensitivity are reduced. Specifically, the number of electric chargesphotoelectrically-converted per pixel, that is, the number of electronsper pixel, is reduced and the saturated electric charge amount(accordingly, saturated signal amount) decreases. This tendencyincreases as the pixel is further miniaturized.

When insulation and isolation based on the LOCOS isolation system or STIisolation system is used as element isolation, a dark current and awhite spot may be caused on an interface between the photodiode servingas the photoelectric conversion element and the insulated and isolatedarea.

It is desirable to provide a solid-state imaging device and a camera inwhich the sensitivity is increased by improving the conversionefficiency when converting electric charges into a signal voltage, whilesuppressing the occurrence of a dark current and a white spot.

According to an embodiment of the present invention, there is provided asolid-state imaging device having arrayed pixels that each include aphotoelectric conversion element and a read transistor for readingelectric charges photoelectrically-converted in the photoelectricconversion element to a floating diffusion portion. An element isolationregion bordering the floating diffusion portion is formed of a shallowtrench element isolation region, and other element isolation regions areformed of an impurity diffusion isolation region.

According to an embodiment of the solid-state imaging device and thecamera of the present invention, since the element isolation regionbordering the floating diffusion portion is formed of the shallow trenchelement isolation region, the capacity of the floating diffusion portionis reduced, so that the conversion efficiency is increased. Since otherelement isolation regions are formed of the impurity diffusion isolationregion, the occurrence of a dark current and a white spot can besuppressed.

According to the embodiment of the solid state imaging device and thecamera of the present invention, it is possible to increase thesensitivity by improving the conversion efficiency while suppressing adark current and a white spot. Accordingly, the solid-state imagingdevice and the camera of the embodiment are suitable for application tothe solid-state imaging device and the camera in which the area of apixel is reduced as the number of pixels is increased.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an example of an arrangement of a MOSimage sensor to which an embodiment of the present invention is applied.

FIG. 2 is a circuit diagram showing an example of a circuit arrangementof a unit pixel.

FIG. 3 is a circuit diagram showing another example of a circuitarrangement of a unit pixel.

FIG. 4 is a diagram showing a solid-state imaging device according to afirst embodiment of the present invention, in particular, showing mainportions of a pixel array portion thereof.

FIG. 5 is a cross-sectional view on the line D-D shown in FIG. 4.

FIG. 6 is a diagram showing main portions of the unit pixel shown inFIG. 4 in an enlarged-scale.

FIG. 7A is a cross-sectional view on the line A-A in FIG. 4; FIG. 7B isa cross-sectional view on the line B-B in FIG. 4; and FIG. 7C is across-sectional view on the line C-C in FIG. 4.

FIGS. 8A and 8B are a plan view and a cross-sectional view respectivelyshowing an example of a gate electrode of a pixel transistor.

FIGS. 9A and 9B are a plan view and a cross-sectional view respectivelyshowing another example of a gate electrode of a pixel transistor.

FIGS. 10A and 10B are a plan view and a cross-sectional viewrespectively showing a further example of a gate electrode of a pixeltransistor.

FIGS. 11A and 11B are a plan view and a cross-sectional viewrespectively showing yet another example of a gate electrode of a pixeltransistor.

FIG. 12 is a diagram showing a solid-state imaging device according to asecond embodiment of the present invention, and in particular, showingmain portions of a pixel array portion thereof.

FIG. 13A is a cross-sectional view on the line A-A in FIG. 12; FIG. 13Bis a cross-sectional view on the line B-B in FIG. 12; and FIG. 13C is across-sectional view on the line C-C in FIG. 12.

FIG. 14 is a cross-sectional view showing a solid-state imaging deviceaccording to a third embodiment of the present invention, and inparticular, showing main portions of a pixel array portion thereof.

FIG. 15 is a circuit diagram showing an example of a pixel sharingcircuit arrangement to which an embodiment of the present invention isapplied.

FIG. 16 is a schematic diagram showing a configuration of a cameraaccording to an embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Conversion efficiency of the photoelectric conversion when convertingelectric charges into a signal voltage has been studied. Specifically,photoelectrically-converted electric charges are converted into avoltage and outputted as a pixel signal from a circuit of the MOS imagesensor. Hence, even when the number of electrons (the amount of electriccharges) per pixel is small, a decrease in the number caused by thereduction in the area of the photodiode can be compensated for if theconversion efficiency that expresses a signal voltage per electriccharge is increased.

Conversion efficiency η is defined by the following equation (1). A unitis μV/e.

$\begin{matrix}{\eta = {\frac{q}{C_{FD}}{G\left\lbrack {\mu\;{V/e}} \right\rbrack}}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

q: amount of electric charge per electron

C_(FD): total capacity relating to a floating diffusion portion

G: gain of source follower

The conversion efficiency η is proportional to the reciprocal number ofthe total capacity relating to the floating diffusion portion andproportional to the gain G of the source follower. Accordingly, theconversion efficiency η increases by increasing the gain and decreasingthe total capacity of the floating diffusion portion. The total capacityof the floating diffusion portion indicates all of the junctioncapacitance of a diffusion layer, which becomes the floating diffusionportion, the gate overlap capacity, the wiring capacity of a wireconnected to the floating diffusion portion and other capacity relatingto the floating diffusion portion. Here, since the gain of the sourcefollower is 1.0 at the maximum and typically about 0.8, it is importantto reduce the total capacity C_(FD) of the floating diffusion portionfor improving the conversion efficiency η.

According to embodiments of a solid-state imaging device and a camera ofthe present invention, the total capacity relating to a floatingdiffusion portion is reduced to improve the conversion efficiency, andtherefore a dark current and a white spot can be suppressed.

The embodiments of the present invention will be described below indetail with reference to the drawings.

FIG. 1 is a block diagram showing an example of a configuration of anamplification solid-state imaging device, such as a MOS (metal-oxidesemiconductor) image sensor, to which an embodiment of the presentinvention is applied. As shown in FIG. 1, a MOS image sensor 10according to the embodiment has an area sensor configuration. The MOSimage sensor 10 includes a unit pixel 11 including a photoelectricconversion element, for example, a photodiode, a pixel array portion 12including the unit pixels 11 arrayed in a two-dimensional matrix, avertical selecting circuit 13, a column circuit 14 serving as a signalprocessing circuit, a horizontal selecting circuit 15, a horizontalsignal line 16, an output circuit 17, a timing generator 18 and others.

Vertical signal lines 121 are wired in the pixel array portion 12 foreach column of the pixels arranged in a matrix. A specific circuitarrangement of the unit pixel 11 will be described later. The verticalselecting circuit 13 includes a shift register and so on. The verticalselecting circuit 13 outputs control signals, such as a transfer signal,to drive a read transistor (hereinafter referred to as a “transfertransistor”, and a read gate electrode is referred to as a “transfergate electrode”) 112 and a reset signal to drive a reset transistor 113for respective rows sequentially. As a result, the respective unitpixels 11 in the pixel array portion 12 are selectively driven forrespective rows.

The column circuit 14 is a signal processing circuit provided for thepixels in the horizontal direction of the pixel array portion 12, thatis, for respective vertical signal lines 121, and includes a S/H (sampleand hold) circuit and a CDS (correlated double sampling) circuit and thelike. The horizontal selecting circuit 15 includes a shift register andso on. The horizontal selecting circuit 15 sequentially selects signalsoutputted from the respective pixels 11 through the column circuit 14and outputs the results to the horizontal signal line 16. Here,horizontal selecting switches are not illustrated in FIG. 1 in order tosimplify the description. The horizontal selecting switches aresequentially turned ON/OFF for respective columns by the horizontalselecting circuit 15.

When the horizontal selecting circuit 15 selectively drives thehorizontal selecting switches, signals of the unit pixels 11sequentially outputted from the column circuit 14 for respective columnsare supplied through the horizontal signal line 16 to the output circuit17, amplified and processed at the output circuit 17 and outputted tothe outside of a device. The timing generator 18 generates varioustiming signals, and drives and controls the vertical selecting circuit13, the column circuit 14 and the horizontal selecting circuit 15 basedon such signals.

FIG. 2 is a circuit diagram showing an example of a circuit arrangementof the unit pixel 11. As shown in FIG. 2, a unit pixel 11A according tothe example includes three pixel transistors of the transfer transistor112, the reset transistor 113 and an amplification transistor 114, inaddition to the photoelectric conversion element, for example, aphotodiode 111. Here, N-channel MOS transistors, for example, are usedas these pixel transistors 112, 113 and 114.

The transfer transistor 112 is connected between the cathode of thephotodiode 111 and the FD (floating diffusion) portion 116 and transferssignal electric charges (herein, electrons) photoelectrically-convertedin the photodiode 111 and accumulated therein to the FD portion 116 uponreceiving a transfer pulse φTRG supplied to the gate thereof.

The drain of the reset transistor 113 is connected to a selection powersupply SELVDD and the source thereof is connected to the FD portion 116.Thus, when a reset pulse φRST is supplied to the gate of the resettransistor 113 before signal electric charges are transferred from thephotodiode 111 to the FD portion 116, the reset transistor 113 resetsthe electric potential of the FD portion 116. The selection power supplySELVDD selectively uses a VDD level and a GND level as a power supplyvoltage.

The amplification transistor 114 has a source follower arrangement inwhich the gate thereof is connected to the FD portion 116, the drainthereof is connected to the selection power supply SELVDD and the sourcethereof is connected to the vertical signal line 121. When the selectionpower supply SELVDD has a VDD level, the amplification transistor 114 isenergized to select the pixel 11A and outputs electric potentialobtained from the FD portion 116 reset by the reset transistor 113 tothe vertical signal line 121 as a reset level. Further, theamplification transistor 114 outputs potential obtained from the FDportion 116 after signal electric charges are transferred as a signallevel to the vertical signal line 121 by the transfer transistor 112.

FIG. 3 is a circuit diagram showing another example of a circuitarrangement of the unit pixel 11. As shown in FIG. 3, a unit pixel 11Baccording to the circuit example is a pixel circuit which includes, inaddition to the photoelectric conversion element, for example, thephotodiode 111, four pixel transistors of the transfer transistor 112,the reset transistor 113, the amplification transistor 114 and theselection transistor 115. Herein, N-channel MOS transistors, forexample, are used as these pixel transistors 112 to 115.

The transfer transistor 112 is connected between the cathode of thephotodiode 111 and the FD (floating diffusion) portion 116 and transferssignal electric charges (herein, electrons) photoelectrically-convertedin the photodiode 111 and accumulated herein to the FD portion 116 inresponse to the transfer pulse φTRG supplied to the gate thereof.

The reset transistor 113 is connected at the drain thereof to the powersupply VDD and connected at the source thereof to the FD portion 116 andresets electric potential of the FD portion 116 when the reset pulseφRST is supplied to the gate thereof before signal electric charges aretransferred from the photodiode 111 to the FD portion 116.

The selection transistor 115 is connected at the drain thereof to thepower supply VDD, connected at the source thereof to the drain of theamplification transistor 114 and turned ON in response to the selectionpulse φSEL supplied to the gate thereof to select the pixel 11B bysupplying the power supply VDD to the amplification transistor 114. Itshould be noted that the selection transistor 115 may be connectedbetween the source of the amplification transistor 114 and the verticalsignal line 121.

The amplification transistor 114 has a source follower arrangement inwhich the gate thereof is connected to the FD portion 116, the drainthereof is connected to the source of the selection transistor 115 andthe source thereof is connected to the vertical signal line 121,respectively. The amplification transistor 114 outputs electricpotential of the FD portion 116 after reset by the reset transistor 113to the vertical signal line 121 as a reset level. Further, theamplification transistor 114 outputs electric potential of the FDportion 116 after signal electric charges are transferred by thetransfer transistor 112 to the vertical signal line 121 as a signallevel.

Next, an embodiment of a pixel array portion according to the presentinvention, which is applied to the above-mentioned pixel array portion12, will be described.

FIG. 4 to FIGS. 7A, 7B and 7C show a solid-state imaging device, in thisembodiment, a CMOS image sensor according to a first embodiment of thepresent invention, and in particular, a first embodiment of a pixelarray portion thereof.

FIG. 4 shows an example of a layout of a pixel array portion 12according to the embodiment of the present invention. In thisembodiment, as shown in FIG. 4, the pixel array portion includes anarray of a plurality of unit pixels 11 formed of a photodiode 22 servingas a photoelectric conversion element and three pixel transistors, thatis, a transfer transistor Tr1, a reset transistor Tr2 and anamplification transistor Tr1. In this example, each of the transistorsTr1 to Tr3 is formed of a n-channel MOS transistor.

As shown in FIGS. 4 and 5 (FIG. 5 is a cross-sectional view on the lineD-D in FIG. 4), a first conductivity-type semiconductor substrate, inthis example, a n-type silicon substrate 20, is provided with a secondconductivity-type, for example, a p-type semiconductor well region 21formed thereon. The photodiode 22 is formed in the p-type semiconductorwell region 21 and includes a n-type semiconductor region (diffusionlayer) 23 which becomes a charge accumulation region and a p-typeaccumulation layer 24 to suppress a dark current on the surface of then-type semiconductor region (diffusion layer) 23.

The transfer transistor Tr1 includes the photodiode 22 as the sourcethereof, the n-type semiconductor region (diffusion layer) 25, whichbecomes the floating diffusion (FD) portion formed in the p-typesemiconductor well region 21, as the drain thereof and the transfer gateelectrode 27 formed through a gate-insulated film 26.

The reset transistor Tr2 includes the n-type semiconductor region 25,which becomes the floating diffusion (FD) portion as the source thereof,a n-type semiconductor region (diffusion layer) 28 formed in the p-typesemiconductor well region 21 as the drain thereof and a reset gateelectrode 29 formed through the gate-insulated film 26.

The amplification transistor Tr3 includes n-type semiconductor regions(diffusion layers) 31 and 28 formed in the p-type semiconductor wellregion 21 as the source and drain thereof and an amplification gateelectrode 32 formed through the gate-insulated film 26.

Then, according to an embodiment of the present invention, inparticular, as shown in FIG. 6 (FIG. 6 is an enlarged diagram of mainportions in FIG. 4) and FIGS. 7A, 7B and 7C (FIG. 7A is across-sectional view on the line A-A in FIG. 6, FIG. 7B is across-sectional view on the line B-B in FIG. 6 and FIG. 7C is across-sectional view on the line C-C in FIG. 6), an element isolationregion bordering the n-type semiconductor region 25 which becomes thefloating diffusion (FD) portion is formed as follows. Specifically then-type semiconductor region 25 is formed by a STI element isolationregion (hereinafter referred to as a “shallow trench element isolationregion”) 36. The shallow trench element isolation region 36 is formedsuch that a trench 34 formed in the substrate 21 is filled with aninsulating film, for example, a silicon oxide film 35, and anotherelement isolation region is formed by a diffusion isolation regionformed of an impurity diffusion portion (hereinafter referred to as an“impurity diffusion isolation region”) 37. In this example, the impuritydiffusion isolation region 37 is formed of a p-type semiconductorregion, that is, an opposite conductivity type to those of the diffusionlayers 25, 28 and 31 of the respective transistors Tr1 to Tr3.

Also, an insulating film 39 having a film thickness equal to that of thegate-insulated film 26 is formed on the whole surface of the shallowtrench element isolation region 36 and the impurity diffusion isolationregion 37. The insulating film 39 on the element isolation regions 36and 37 is substantially formed of the gate-insulated film 26 of thetransistor, that is, an extended portion of the gate-insulated film 26.The insulating film other than the insulating film 39 equal to thegate-insulated film is not formed on the element isolation regions 36and 37. Accordingly, a whole area from the active region of thetransistor to the element isolation regions 36 and 37 becomes aplanarized surface. Part of the transfer gate electrode 27, the resetgate electrode 29 and the amplification gate electrode 32 of each of thetransistors Tr1 to Tr3 is extended from the channel region to theimpurity diffusion isolation region 37.

The respective gate electrodes 27, 29 and 32 of the respectivetransistors Tr1 to Tr3 include first portions corresponding to channelregions 41, 42 and 43 that are active regions and second portionsextended from the channel region to the element isolation region (thatis, impurity diffusion isolation region) 37, which are made of differentmaterials. While the gate electrodes 27, 29 and 32 are made ofpolysilicon amorphous silicon, in this example, polysilicon impuritiesdoped into the first portions are made differently from the secondportions. FIGS. 8A and 8B to FIGS. 11A and 11B show respective examples.Here, the symbol S denotes a source region, the symbol D denotes a drainregion and the reference numeral 37 denotes the diffusion isolationregion.

In the examples shown in FIGS. 8A and 8B, a first portion of the gateelectrode [27, 29, 32] is formed of n-type impurity doped polysiliconand a second portion 47 is formed of p-type impurity doped polysilicon(first portion/second portion are formed of n-type impurity dopedpolysilicon/p-type impurity doped polysilicon).

In the examples shown in FIGS. 9A and 9B, the first portion 46 of thegate electrode [27, 29, 32] is formed of p-type impurity dopedpolysilicon and the second portion 47 is formed of n-type impurity dopedpolysilicon (first portion/second portion are formed of p-type impuritydoped polysilicon/n-type impurity doped polysilicon).

In the examples shown in FIGS. 10A and 10B, the first portion 46 of thegate electrode [27, 29, 32] is formed of n-type impurity dopedpolysilicon and the second portion 47 is formed of non-doped polysilicon(first portion/second portion are formed of n-type impurity dopedpolysilicon/non-doped polysilicon).

In the examples shown in FIGS. 11A and 11B, the first portion 46 of thegate electrode [27, 29, 32] is formed of p-type impurity dopedpolysilicon and the second portion 47 is formed of non-doped polysilicon(first portion/second portion are formed of p-type impurity dopedpolysilicon/non-doped polysilicon).

According to the MOS image sensor of a first embodiment of the presentinvention, the element isolation region bordering the n-typesemiconductor region 25 which becomes the floating diffusion portion isformed by the shallow trench element isolation region 36 in which theinsulating film 35 is filled into the trench 34. A capacity formedbetween the n-type semiconductor region 25 of the floating diffusionportion and the substrate is reduced. As a result, the total capacityC_(FD) of the floating diffusion portion can be reduced and theconversion efficiency when photoelectrically-converted electric chargesare converted into a signal voltage can be increased. Accordingly, evenwhen the number of photoelectrically-converted electric charges, thatis, the number of electrons, is reduced as the pixel is miniaturized, ahigh conversion efficiency can be obtained so that the sensitivity ofthe MOS image sensor can be improved. On the other hand, since theelement isolation region at the region other than the region borderingthe floating diffusion portion is formed of the p-type impuritydiffusion isolation region 37, the occurrence of a dark current and awhite spot can be suppressed.

Also, since only the insulating film having the film thickness equal tothat of the gate-insulated film, in this example, the same insulatingfilm 39 as the gate-insulated film 26 is formed on the element isolationregions 36 and 37, the gate electrode can be prevented from overlappingthe element isolation regions 36 and 37. Thus, even when the pixel isminiaturized increasingly, the structure can be simplified withoutmaking the structure on the surface complicated.

Further, if the gate electrodes 27, 29 and 32 of the respectivetransistors Tr1 to Tr3 have the combined structures in which the firstportions 46 corresponding to the channel regions and the second portions47 corresponding to the impurity diffusion isolation regions 37 areformed of different materials, that is, n-type impurity doped materials,p-type impurity doped materials and non-doped materials, then even whena gate voltage is applied to the first portion, a gate voltage is notapplied to the second portion. That is, since a pn-junction is formed ata boundary between the first and second portions 46 and 47, or thesecond portion 47 is formed of the non-doped material and has a highresistance to act substantially as an insulating material, a gatevoltage is not applied to the second portion 47. Accordingly, aparasitic MOS transistor that uses the second portion 47 as a parasiticgate can be prevented from being formed. As a result, it is possible toprevent electric charges from being leaked from the channel region tothe channel side (impurity diffusion isolation region 37) and to preventelectric charges from being leaked into the adjacent pixels reliably.

FIG. 12 and FIGS. 13A to 13C show a solid-state imaging device, in thisexample, a MOS image sensor according to a second embodiment of thepresent invention, and in particular, a second embodiment of a pixelarray portion thereof. FIG. 12 and FIGS. 13A to 13C correspond to FIG. 6and FIGS. 7A to 7C according to the above-described first embodiment ofthe present invention, respectively. The rest of arrangement is similarto those shown in FIGS. 4 and 5. In FIG. 12 and FIGS. 13A to 13C,elements and parts identical to those of FIG. 6 and FIGS. 7A to 7C aredenoted by identical reference numerals.

Also, in the second embodiment of the present invention, similarly tothe above description made with reference to FIGS. 4 and 5, thesolid-state imaging device includes an array of a plurality of unitpixels 11 including the photodiode 22 which is the photoelectricconversion element and the three pixel transistors, that is, the readtransistor (hereinafter referred to as a “transfer transistor”) Tr1, thereset transistor Tr2 and the amplification transistor Tr3. In thisembodiment, each of the transistors Tr1 to Tr3 is composed of an-channel MOS transistor.

Also, as similarly shown in FIGS. 4 and 5, the first conductivity-typesemiconductor substrate, that is, the n-type silicon substrate 20 isprovided with the second conductivity-type, for example, the p-typesemiconductor well region 21 formed thereon. The photodiode 22 is formedin the p-type semiconductor well region 21 and includes the n-typesemiconductor region (diffusion layer) 23 which becomes the chargeaccumulation region and the p-type accumulation layer 24 to suppress adark current on the surface of the n-type semiconductor region(diffusion layer) 23.

The transfer transistor Tr1 includes the photodiode 22 as the sourcethereof, the n-type semiconductor region (diffusion layer) 25, whichbecomes the floating diffusion (FD) portion formed in the p-typesemiconductor well region 21, as the drain thereof and the transfer gateelectrode 27 formed through the gate-insulated film 26.

The reset transistor Tr2 includes the n-type semiconductor region 25,which becomes the floating diffusion (FD) portion, as the sourcethereof, the n-type semiconductor region (diffusion layer) 28 formed inthe p-type semiconductor well region 21 as the drain thereof and thereset gate electrode 29 formed through the gate-insulated film 26.

The amplification transistor Tr3 includes the n-type semiconductorregions (diffusion layers) 31 and 28 formed in the p-type semiconductorwell region 21 as the source and drain thereof and the amplificationgate electrode 32 formed through the gate-insulated film 26.

Then, according to the second embodiment of the present invention, inparticular, as shown in FIG. 12 and FIGS. 13A to 13C (FIG. 13A is across-sectional view on the line A-A in FIG. 12, FIG. 13B is across-sectional view on the line B-B in FIG. 12 and FIG. 13C is across-sectional view on the line C-C in FIG. 12), the element isolationregion is formed as follows. Accordingly, the element isolation regionextends from the region bordering the n-type semiconductor region 25which becomes the floating diffusion (FD) portion along the transferchannel region to the region extending to a portion under the transfergate electrode. The element isolation region is formed by a shallowtrench element isolation region 36 in which the trench 34 formed in thesubstrate 21 is filled with the insulating film, for example, thesilicon oxide film 35. The other element isolation region is formed ofthe impurity diffusion isolation region 37 made of an impurity diffusionportion. In this example, the impurity diffusion isolation region 37 isformed of a p-type semiconductor region, that is, a conductivity type,opposite to those of the diffusion layers 25, 28 and 31 of therespective transistors Tr1 to Tr3.

An end portion of the shallow trench element isolation region 36extended under the transfer gate electrode is not in contact with then-type semiconductor region 23 which is the charge accumulation regionof the photodiode, and instead, the impurity diffusion isolation region37 is provided between the end of the extended portion of the shallowtrench element isolation region 36 and the photodiode 22.

Also, as shown in FIGS. 13A to 13C, an insulating film 39 having a filmthickness equal to that of the gate-insulated film 26 is formed on thewhole surface of the shallow trench element isolation region 36 and theimpurity diffusion isolation region 37. The insulating film 39 on theelement isolation regions 36 and 37 is substantially formed of thegate-insulated film 26 of the transistor, that is, the extended portionof the gate-insulated film 26. The insulating film other than theinsulating film 39 equal to the gate-insulated film 26 is not formed onthe element isolation regions 36 and 37. Accordingly, the whole areafrom the active region of the transistor to the element isolationregions 36 and 37 is thoroughly formed as a planarized surface. A partof the transfer gate electrode 27, the reset gate electrode 29 and theamplification gate electrode 32 of the respective transistors Tr1 to Tr3is extended from the channel region to the impurity diffusion isolationregion 37.

According to the second embodiment of the present invention, in therespective gate electrodes 27, 29 and 32 of the respective transistorsTr1 to Tr3, the first portions corresponding to channel regions 41, 42and 43 which are active regions and the second portions extended fromthe channel region to the impurity diffusion isolation region 37 can bemade of different materials similarly to the above-described respectiveexamples shown in FIG. 8 to FIGS. 11A to 11C.

According to the MOS image sensor of the second embodiment of thepresent invention, the element isolation region bordering the n-typesemiconductor region 25 which is the floating diffusion portion isformed by the shallow trench element isolation region 36 in which theinsulating film 35 is filled into the trench 34. As a result, the totalcapacity C_(FD) of the floating diffusion portion can be reduced and theconversion efficiency when photoelectrically-converted electric chargesare converted into a signal voltage can be increased. Accordingly, evenwhen the number of photoelectrically-converted electric charges, thatis, the number of electrons, is reduced as the pixel is miniaturized, ahigh conversion efficiency can be obtained so that the sensitivity ofthe MOS image sensor can be improved.

Further, since a part of the shallow trench element isolation region 36is extended under the transfer gate electrode 27, electric charges canbe read readily from the photodiode 22 to the floating diffusion portion25. That is, if the element isolation region at the side of the transferchannel is formed of the p-type impurity diffusion isolation region 37,p-type impurities may be diffused readily in the annealing process ofthe manufacturing process from the impurity diffusion isolation regionto the portion under the transfer gate electrode, that is, the transferchannel region. When the p-type impurities are diffused into thetransfer channel region, a threshold voltage Vt of the transfertransistor Tr1 is increased effectively, which may cause difficulty inreading electric charges. However, according to the second embodiment ofthe present invention, the shallow trench element isolation region 36can prevent p-type impurities from being diffused from the impuritydiffusion isolation region to the transfer channel region in theannealing process, so that the threshold voltage Vt can be preventedfrom being raised.

Also, since a part of the shallow trench element isolation region 36 isformed extending under the transfer gate electrode 27, it is possible toprevent electric charges from being leaked from the transfer channelregion to the impurity diffusion isolation region 37 at the side of thechannel.

On the other hand, since the element isolation region in the regionother than the region extended from the portion bordering the floatingdiffusion portion to a portion under the part the transfer gateelectrode is formed of the p-type impurity diffusion isolation region37, it is possible to suppress the occurrence of a dark current and awhite spot.

Further, when the first portion 46 in which the gate electrodes 27, 29and 32 of the respective transistors Tr1 to Tr3 correspond to thechannel regions and the second portion 47 corresponding to the impuritydiffusion isolation region 37 are formed of different materials asmentioned above, it is possible to prevent the parasitic MOS transistorhaving the second portion 47 as the parasitic gate from being formed.Also, it is possible to reliably prevent electric charges from beingleaked from the channel region to the channel side (impurity diffusionisolation region 37), or it is possible to reliably prevent electriccharges from being leaked to the adjacent pixels.

According to the first and second embodiments of the present invention,the whole surface of the shallow trench element isolation region 36 andthe impurity diffusion isolation region 37 is planarized by forming theinsulating film 39 having the film thickness equal to that of thegate-insulated film 26. However, an embodiment of the present inventionis not limited thereto and, as shown in a third embodiment of thepresent invention shown in FIG. 14, an insulating film 48 thicker thanthe gate-insulated film 26, for example, a silicon oxide film, can beformed on the impurity diffusion isolation region 37. The rest ofarrangement is similar to that of the first embodiment of the presentinvention or that of the second embodiment of the present invention.

In the case of the above-described arrangement, differently from thearrangement shown in FIGS. 8A and 8B to FIGS. 11A and 11B, the firstportion corresponding to the active region and the second portioncorresponding to the element isolation region may be made of the samematerial. Since the insulating film 48 having the large thickness isformed on the impurity diffusion isolation region 37, even when the gateelectrode overlaps the thick insulating film 48, it is possible toprevent a parasitic MOS transistor from being formed.

Also, according to the third embodiment of the present invention shownin FIG. 14, the element isolation region bordering at least the floatingdiffusion portion is formed of the shallow trench element isolationregion 36. Accordingly, the conversion efficiency can be enhanced, andthe sensitivity can be increased even when the pixel is miniaturized.Further, since the other element isolation region is formed of theimpurity diffusion isolation region 37, it is possible to suppress theoccurrence of a dark current and a white spot.

Embodiments of the present invention are suitable for application to aMOS image sensor having an arrangement in which pixel transistors otherthan the transfer transistor is shared with a plurality of pixels (forexample, two pixels, four pixels, etc.). This arrangement hereinafterwill be referred to as a “pixel sharing arrangement”. According to thepixel sharing MOS image sensor, electric charges from two photodiodesare alternately read at one floating diffusion portion depending on thelayout. For example, when a solid-state imaging device is of afour-pixel sharing, two floating diffusion portions are formed and thetwo floating diffusion portions are electrically connected by wiring.For this reason, a total capacity of the floating diffusion portions maybe increased. Therefore, if the element isolation region in which theshallow trench element isolation region and the impurity diffusionisolation region are combined is applied to the solid-state imagingdevice as an element isolation region, then it is possible to improveconversion efficiency by reducing the total capacity of the floatingdiffusion portions.

FIG. 15 shows an example of an equivalent circuit of a MOS image sensorin which pixel transistors are shared with four pixels. As shown in FIG.15, an equivalent circuit according to this embodiment includes fourphotodiodes PD1, PD2, PD3, PD4, four transfer transistors TrG1, TrG2,TrG3, TrG4, two floating diffusion portions FD1, FD2, a shared resettransistor TrRST, an amplification transistor TrAMP and a selectiontransistor TrSEL.

The first and third photodiodes PD1 and PD3 are connected through thetransfer transistors TrG1 and TrG3 to the first floating diffusionportion FD1. Also, the second and fourth photodiodes PD2 and PD4 areconnected through the transfer transistors TrG2 and TrG4 to the secondfloating diffusion portion FD2. Transfer wirings 51, 52, 53 and 54 forsupplying transfer pulses are respectively connected to the gates of thefirst to fourth transfer transistors TrG1 to TrG4.

The first and second floating diffusion portions FD1 and FD2 areconnected in common to the gate of the amplification transistor TrAMPand to the source of the reset transistor TrRST. A power supply wiring(VDD) 56 is connected to the drain of the reset transistor TrRST and thedrain of the amplification transistor TrAMP. A reset wiring 57 forsupplying a reset pulse is connected to the gate of the reset transistorTrRST.

The source of the amplification transistor TrAMP is connected to thedrain of the selection transistor TrSEL, the source of the selectiontransistor TrSEL is connected to a vertical signal line 59 and the gateof the selection transistor TrSEL is connected to a selection wiring 58for supplying a selection pulse.

In the above-described circuit arrangement, electric chargesphotoelectrically-converted at the respective photodiodes PD1 to PD4 aresequentially read to the corresponding first and second floatingdiffusion portions FD1 and FD2 with a time difference, converted into apixel signal at the amplification transistor TrAMP and outputted to thevertical signal line 59. The electric charges read to the first andsecond floating diffusion portions FD1 and FD2 are converted into thepixel signal and reset through the reset transistor TrRST.

According to a fourth embodiment of the present invention, the MOS imagesensor includes the pixel array portion in which the equivalent circuitof the four-pixel sharing structure shown in FIG. 14 is arrayed. Theelement isolation regions bordering the first and second floatingdiffusion portions FD1 and FD2 are formed of the shallow trench elementisolation regions, and the other element isolation regions are formed ofthe impurity diffusion isolation regions, thereby obtaining an elementisolation structure similar to those of the above-described first andsecond embodiments of the present invention.

According to the fourth embodiment of the present invention, since thefirst and second floating diffusion portions FD1 and FD2 areelectrically connected in the four-pixel sharing arrangement, thecapacity at the floating diffusion portions FD1 and FD2 increases.However, since the element isolation regions bordering the floatingdiffusion portions FD1 and FD2 are formed of the shallow trench elementisolation regions, the capacity at the floating diffusion portion can bereduced, and hence the conversion efficiency can be improved.Accordingly, based on a combination of the shallow trench elementisolation region and the impurity diffusion isolation region, it ispossible to increase the sensitivity by improving the conversionefficiency while suppressing the occurrence of a dark current and awhite spot.

According to the MOS image sensor, the reset transistor constituting thepixel may be formed with a distance from the floating diffusion portion.In this arrangement, a diffusion layer that is to be the floatingdiffusion portion and the source region of the reset transistor areconnected by wiring. If an embodiment of the present invention isapplied to the arrangement, as shown in the first and second embodimentsof the present invention, the element isolation region of the region atleast bordering the floating diffusion portion is formed of the shallowtrench element isolation region. In addition, the element isolationregion bordering the circumference of the source region (diffusionlayer) of the independently formed reset transistor also is formed ofthe shallow trench element isolation region. The other element isolationregions are formed of the impurity diffusion isolation regions.

According to the above-mentioned fifth embodiment of the presentinvention, the sensitivity can be improved by increasing the conversionefficiency while suppressing the occurrence of a dark current and awhite spot.

While the n-channel MOS transistor is applied as respective pixeltransistors to the solid-state imaging device according to theabove-described embodiments, the present invention is not limitedthereto, and a p-channel MOS transistor can be applied to thesolid-state imaging device as a pixel transistor. In the case of then-channel MOS transistor, the n-type is set to the first conductivitytype and the p-type is set to the second conductivity type in theabove-mentioned embodiments. In the case of the p-channel MOStransistor, the p-type is set to the first conductivity type and then-type is set to the second conductivity type. That is, the n-channeland the p-channel have opposite conductivity types.

Also, the above-described embodiments of the present invention areapplied, for example, to the area sensor in which pixels are arrayed ina two-dimensional matrix. The present invention is not limited to theapplication to the area sensor and can be applied to a linear sensor(line sensor) in which the above-described pixels are linearly arrayedone-dimensionally.

FIG. 16 is a schematic cross-sectional view showing a camera accordingto an embodiment of the present invention. The camera according to theembodiment of the present invention is a video camera capable ofcapturing still images or moving images, for example.

As shown in FIG. 16, the camera according to the embodiment of thepresent invention includes a MOS image sensor 10, an optical system 210,a shutter device 211, a drive circuit 212 and a signal processingcircuit 213.

The optical system 210 focuses image light (incident light) reflectedfrom an object on an imaging screen of the MOS image sensor 10. As aresult, signal electric charges are accumulated in the MOS image sensor10 for a specific period.

The shutter device 211 is configured to control a light irradiationperiod and a light shaded period for the MOS image sensor 10.

The drive circuit 212 is configured to supply drive signals to controltransfer operations of the MOS image sensor 10 and shutter operations ofthe shutter device 211. The MOS image sensor is configured to transfersignals upon receiving a drive signal (timing signal) supplied from thedrive circuit 212. The signal processing circuit 213 is configured tocarry out various kinds of signal processing. An image signal obtainedafter signal processing is stored in a storage medium, such as a memory,or outputted to a monitor.

The above-described solid-state imaging device according to theembodiments of the present invention, specifically, the MOS imagesensor, is suitable for the application to a solid-state imaging devicemounted on mobile devices, such as a mobile phone unit with a camera anda PDA.

In particular, according to the embodiments of the present invention,the conversion efficiency can be improved while suppressing theoccurrence of a dark current and a white spot, if the area of thephotodiode, which is the photoelectric conversion element, isminiaturized along with the pixel size being reduced as the number ofpixels is increased.

It should be understood by those skilled in the art that variousmodifications, combinations, subcombinations and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

What is claimed:
 1. A solid-state imaging device comprising: a pixelunit including a plurality of pixels on a substrate, wherein each of thepixels includes a photoelectric conversion element and a transfertransistor, the transfer transistor being configured to transfer chargesgenerated in the photoelectric conversion element to a floatingdiffusion portion, a diffusion isolation region is formed between thepixels, a trench isolation structure contacts the floating diffusionportion on a first side of the trench isolation structure and contactsthe diffusion isolation region on a second side of the trench isolationstructure opposite the first side, and the trench isolation structurecontinuously extends to a portion beneath and covered by a gate of thetransfer transistor from a portion bordering the floating diffusionportion and is not in contact with the photoelectric conversion portion.2. The solid-state imaging device according to claim 1, wherein thediffusion isolation region is different shape structure from the trenchisolation region.
 3. The solid-state imaging device according to claim1, wherein the trench isolation structure is formed by a shallow trenchstructure.
 4. The solid-state imaging device according to claim 1,wherein the diffusion isolation region is interposed between an end ofan extended portion of the trench isolation structure under a read gateelectrode and the photoelectric conversion element.
 5. The solid-stateimaging device according to claim 1, wherein a planarized insulatingfilm having a film thickness equal to that of a gate-insulated film isformed on the trench isolation structure and the diffusion isolationregion.
 6. The solid-state imaging device according to claim 1, whereinan insulating layer thicker than the gate-insulated film is formed onthe diffusion isolation region.
 7. The solid-state imaging deviceaccording to claim 1, further comprising a reset transistor in thepixel, wherein a source region of the reset transistor is the same asthe floating diffusion portion.
 8. The solid-state imaging deviceaccording to claim 1, wherein a plurality of the arranged pixels sharepixel transistors other than the transfer transistor.
 9. The solid-stateimaging device according to claim 8, wherein the pixel transistorsinclude at least a reset transistor.
 10. The solid-state imaging deviceaccording to claim 1, wherein the diffusion isolation region is formedby an impurity diffusion isolation region.
 11. The solid-state imagingdevice according to claim 3, wherein the shallow trench structure isfilled with an insulating film.
 12. The solid-state imaging deviceaccording to claim 1, wherein a conductivity of the substrate isopposite to a conductivity of the diffusion isolation region.
 13. Thesolid-state imaging device according to claim 1, wherein a semiconductorwell region is formed on the substrate, the photoelectric conversionelement and the floating diffusion portion being formed in thesemiconductor well region, the semiconductor well region being of athird conductivity.
 14. The solid-state imaging device according toclaim 13, wherein a portion of the semiconductor well region is formedbetween the diffusion isolation region and the substrate.
 15. Thesolid-state imaging device according to claim 13, wherein a conductivityof the substrate is opposite to the third conductivity.
 16. Thesolid-state imaging device according to claim 1, wherein a gateelectrode of the transfer transistor includes a first portion and asecond portion, the first portion of the gate electrode being of afourth conductivity and the second portion of the gate electrode beingof a fifth conductivity.
 17. The solid-state imaging device according toclaim 16, wherein the fourth conductivity is opposite to the fifthconductivity.
 18. The solid-state imaging device according to claim 16,wherein the first portion of the gate electrode is formed of a firstimpurity doped polysilicon and the second portion of the gate electrodeis formed of a second impurity doped polysilicon.
 19. The solid-stateimaging device according to claim 16, wherein the first portion of thegate electrode is above the diffusion isolation region in a depthdirection of the substrate, and the second portion of the gate electrodeis above a transfer channel of the transfer transistor.
 20. Thesolid-state imaging device according to claim 19, wherein the firstportion of the gate electrode is n-type doped and the second portion ofthe gate electrode is p-type doped.
 21. The solid-state imaging deviceaccording to claim 16, wherein the second portion of the gate electrodeis formed of non-doped polysilicon.
 22. The solid-state imaging deviceaccording to claim 1, further comprising a reset transistor in thepixel, wherein the trench isolation structure contacts the resettransistor.